1. Field of the Present Invention
The present invention relates to an organic electroluminescent element and a display device using the same which can be effectively applied to a surface light source for full color display, backlight, illumination light sources and the like; or a light source array for printers, and the like.
2. Description of the Related Art
An organic electroluminescence element (hereinafter, referred to as an “organic EL element” in some cases) is composed of a light emitting layer or a plurality of organic layers containing a light emitting layer, and a pair of electrodes sandwiching the organic layers. The organic EL element is an element for obtaining luminescence by utilizing at least either one of luminescence from excitons each of which is obtained by recombining an electron injected from a cathode with a positive hole injected from an anode to produce exciton, or luminescence from exciton of other molecules produced by energy transmission from the above-described excitons.
Heretofore, an organic EL element has been developed by using a laminate structure from integrated layers in which each layer is functionally differentiated, whereby brightness and device efficiency are remarkably improved. For example, it is described in “Science”, vol. 267, No. 3, page 1332; 1995 that a two-layer laminated type device obtained by laminating a positive hole-transport layer and a light emitting layer also functioning as an electron-transport layer; a three-layer laminated type device obtained by laminating a positive hole-transport layer, a light emitting layer, and an electron-transport layer; and a four-layer laminated type device obtained by laminating a positive hole-transport layer, a light emitting layer, a positive hole-blocking layer, and an electron-transport layer have been frequently disclosed.
Conventionally, layers disposed between a light emitting layer and electrodes are generally designed to lower an energy barrier at the interface therebetween for the purpose of allowing positive holes and electrons to smoothly move from an anode and a cathode, respectively. For example, light emitting materials having a relatively low Ip value (ionization potential) are preferably used for a light emitting layer to lower the barrier against positive holes supplied by an anode at an interface between a positive hole transport layer and the light emitting layer. Further, light emitting materials having a relatively high Ea value (electron affinity) are preferably used for a light emitting layer to lower the barrier against electrons supplied by a cathode at an interface between an electron transport layer and the light emitting layer. Such designs have been helpful in reducing driving voltages and improving luminescent efficiency to a certain degree. However, large improvement in luminescent efficiency could not be expected by those means.
Japanese Patent Application Laid-Open (JP-A) No. 2005-38763 discloses that, by a configuration in which a plurality of light emitting layers each containing the same light emitting materials or fluorescent light emitting materials having similar color is laminated in series, element durability is improved. However, large improvement in luminescent efficiency could not be expected.
On the other hand, it is known that theoretical luminescent efficiency is increased by about four times by using a phosphorescence-irradiating material as a light emitting material for an organic electroluminescent element. For example, Japanese National Publication No. 2004-522276 discloses a white luminescent element using a phosphorescence light emitting material. In particular, an organic electroluminescent (EL) element using the phosphorescence light emitting material is attracting attention as a promising luminescent element for a passive matrix organic EL display by making use of a residual image effect. However, it has been revealed that the organic EL element using the phosphorescence light emitting material may cause a specific phenomenon referred to as “Roll-off” in which a radical deterioration of luminescent efficiency occurs in a high current density region of emission. Analysis of the causes and research for means of improving the trouble have been tried from various approaches. However, effective means have not yet been discovered. Therefore, it is desired to further develop improvement means for the organic EL element.